X-ray images produced by non-tomographic techniques are often difficult to interpret and may fail to provide needed information about a medical patient or an inanimate object that is being examined for structural flaws. Data originating from a specific internal region of particular interest may be obscured by overlapping or superimposed imaging of other regions that are forward from or behind the region of interest.
The more recently developed tomographic X-ray imaging techniques are not subject to the above discussed disadvantage. Computer aided tomography can generate a cross sectional depiction of a single plane that is essentially free of data arising from other planes within the subject. Variations of X-ray absorbency between different areas of the imaged plane are made apparent without ambiguity as to location and with much greater clarity than is usually realizable with older techniques.
Most prior tomographic X-ray installations require a bulky, elaborate and costly mechanical scanning system. Installations of this kind have an X-ray source which directs a narrow X-ray beam to a detector at the opposite side of the subject. The source and detector are jointly translated relative to the subject, or the subject itself may be translated, so that the X-ray beam cuts across a plane within the subject that is to be imaged. A single translation of this kind cannot provide a meaningful tomographic or sectional image. The location of points within the plane where a change of X-ray absorbency was detected can be determined with respect to one coordinate but not with respect to the orthogonal coordinate. Consequently it is necessary to turn the source and detector angularly relative to the subject and repeat the translation. The location of the points in both coordinates than becomes determinable by data processing operations comparable to triangulation.
As a practical matter it is usually necessary, in such installations, to perform a large number of translations of the source and detector alternated with a large number of angular repositionings of such components in order to generate an image of desirable resolution and clarity. The mechanisms which enable the source and detector or the subject to be traveled through this repetive combination of linear and angular motions accounts for a considerable part of the bulk, complexity and cost of such installations. The mechanical positioning and scanning structure becomes even more complex if sectional images of more than one plane or of oblique planes are to be generated from a single scanning sequence.
Disadvantages of scanning X-ray installations of the above discussed kind are not limited to size, complexity and cost. An undesirably long period of time is required to perform the mechanical scanning operations. This limits productivity and prolongs the radiation exposure of the subject. The effects of scattered X-rays decrease resolution in a tomographic image and long exposure times aggravate such image degradation.
The problems discussed above are alleviated to a considerable extent by another form of tomographic X-ray scanning system described in my prior U.S. Pat. No. 4,144,457. In the method and apparatus described in that prior patent, the X-ray source has an electron beam which is electrostatically or magnetically deflected to establish a moving X-ray origin point at a broad target plate. Thus the translation portion of the scanning operation is accomplished electronically without necessarily requiring physical movement of the source and detector or the subject for those portions of the scanning operation. Angular motion of the source and detector or the subject continues to be necessary between electronic translations but the mechanism for the purpose can be relatively compact and simple as only simple rotational motion is needed.
The apparatus of my above identified prior patent can be mechanically simpler, more compact and less costly than the wholly mechanical scanning systems which have been hereinbefore discussed. Electronic scanning can be conducted more rapidly than mechanical scanning thereby increasing productivity, decreasing radiation exposure of the subject and with a reduction of image degradation from scattered X-rays.
The above discussed advantages of electronic scanning would become even more pronounced if it were possible to generate a tomographic image without necessarily relying on any relative movement of the source and detector or the subject. Heretofore it has appeared that it is not possible to extract the data that is needed for generating a tomographic iamge in the absence of physical repositionings of the source and detector or subject during the course of scanning operations.
The present invention is directed to overcoming one or more of the problems discussed above.